In audio-visual display systems, the synchronization of audio output with video images (“AV sync”) is an important element for a satisfactory display. The human sensitivity to AV sync is highly variable, but generally needs to be less than ±80 msec for “good” performance. That is, if the audio is not synchronized with the video images to within ±80 msec, the results will be unsatisfactory to viewers. It is generally believed that a high-quality system should strive for video to lead the audio by <40 msec. In extreme cases, the requirements can be as stringent as 5 msec, such as for animation or live action designed to highlight music.
Synchronization of audio and video signals in consumer AV systems is a relatively new problem because of the increasing popularity of AV systems that use separate audio and video output components. New video display devices, such as digital projectors, have increasingly complex video processing pipelines that result in large amounts of delay. In addition, the amount of delay can vary significantly, depending on the type of processing to be done. For example, interlaced composite analog video requires much different processing than progressive-scan digital HDTV. However, users may often make use of different types of input to the display, so that the resulting video delay will vary. Unfortunately, if the user has compensated for the video delay in an external audio system (as is becoming increasingly common), this compensation may not be correct for all video input types. Additionally, there is no way for the audio delay to be automatically adjusted to match the changing video delay. Consequently, a user can become frustrated because their system does not perform consistently well. Additionally, given the stringent requirements for AV sync mentioned above, it is probably not possible to select a single amount of audio delay that can “center” the delay to make performance acceptable for all video input types.
Audio delay capabilities are frequently added to a home-theater setup as a stand-alone box, or as a feature integrated into an audio amplifier. In such a case, however, providing a proper video signal delay to match the externally-created audio delay is required. Additionally, some television sets with integrated sound systems may have audio delay built-in to compensate for the video delay. Nevertheless, whether an AV system comprises separate audio and video components, or is an integral system, a better system for maintaining AV sync is desirable.